Within the electronics industry, soldering remains the technique of choice for bonding the leads of an electronic component to a corresponding pattern of metallized areas on a printed circuit board. In carrying out the soldering process, a quantity of solder flux is often applied either to the component leads, or more frequently, to the metallized areas on the circuit board. The flux, which is generally comprised of an activator, a solid vehicle, and a solvent, acts as both a wetting and cleaning agent to enhance the quality of the solder bond between the lead and metallized area on the circuit board. In the past, solder flux has been applied, by brushing, foaming, spraying, or alternatively, by producing a flux wave across which the circuit board is passed.
Traditionally, rosin-based fluxes, that is, fluxes which employ rosin, have been preferred because of their mild activity, as compared with more chemically active water-soluble fluxes. However, rosin-based fluxes incur the disadvantage that such fluxes leave residues which interfere with testing of the circuit board. Usually, the only way to remove such residues is by cleaning the circuit board with chlorofluorocarbons (CFC's), which are deleterious to the environment, or with harsh detergents which often must be treated before discharge into a municipal sewage system.
In an effort to overcome the cleaning difficulties associated with conventional rosin-fluxes, several manufacturers have introduced "low-solids" fluxes, which are a liquid flux mixture containing very small amounts of solids vehicle. Such low-solids fluxes have been found to leave much reduced amounts of residues, small enough so that testing of the circuit board is not adversely affected even without cleaning. However, the disadvantage of such low-solids fluxes is that unless applied in a very controlled fashion, the surface insulation resistance of the board may vary and thus fall below acceptable limits.
In U.S. Pat. Nos. 4,821,948 and 4,871,105, both issued in the names of John R. Fisher, Leslie A. Guth and James A. Mahler, and both assigned to AT&T, there is disclosed an apparatus for spraying a controlled amount of low-solids flux on a circuit board to avoid the aforementioned difficulty. The Fisher et al. apparatus employs an ultrasonic nozzle which atomizes the liquid low-solids flux into tiny droplets that are injected into a laminar gas stream directed through a horn towards the circuit board to deposit the flux thereon. While the Fisher et al. apparatus has proven effective to spray a controlled amount of low solids on a circuit board, the ultrasonic nozzle employed within the apparatus is relatively expensive. Moreover, the horn often becomes clogged, requiring manual cleaning.
Thus, there is a need for a less costly and more reliable apparatus for applying low-solids flux.